With known air duct sections of this type as shown in German Pat. No. 2,203,560, the supporting arms of the connecting flanges, form fitted in one piece, are fastened on the outside of the duct walls by spot welding. This additional procedure not only increases the cost of manufacturing but, in addition, the spot welded points are also susceptible to corrosion and must, preferably, be coated with an anticorrosive agent. Moreover, the attained fixing of the supporting arm is only at these points and, thus, not uniformly secure over the entire length of the profile.
Furthermore, air duct sections with flange profiles are known as shown in German Pat. No. 1,452,854 which have only one flange projecting at right angles from the duct wall, the outer edge of said flange is bent back at right angles parallel to the duct wall. The opposite bent back edges are pressed together by a profile clamp which is to be slipped on. Of course, with this specific embodiment, one does not have to spot weld, however, the slipping on of the profile clamp onto the bent back edges is extremely troublesome, in particular, then when the bent back edges are easily damaged during transportation and at the construction site, which occurs very frequently. Moreover, the stability of air ducts having connection flanges of this type is unsatisfactory, since the canted flanges at the ends of the air duct sections do not offer an adequate stiffening. Problems also arise during assembly if there is insufficient room for slipping on the profile clamps.